Cameras and other image recording devices often capture images with one or more image sensors, such as a charge-coupled device (CCD) image sensor or a complementary metal-oxide-semiconductor (CMOS) image sensor. Images are captured by converting light into electrical signals. An array of photodetectors accumulates photo-generated charge (e.g., electrons) in response to light striking the photodetectors. The amount of charge accumulated by each photodetector represents the intensity of the light received by that photodetector. The charge accumulated in all of the photodetectors collectively forms the image.
A CMOS image sensor can be configured as a frontside illuminated (FSI) or as a backside illuminated (BSI) image sensor. A FSI image sensor positions one or more patterned metal layers over the substrate containing an array of photodetectors. The metal layer includes the signal lines that connect the various electrical components in the pixels to operational and power supply circuitry located outside of the array. However, a FSI configuration means light must first pass through the metal layer before it is detected by the photodetectors. The metal layer can interfere with light transmission by reflecting some of the light as the light passes through the metal layer, which can reduce the amount of light detected by the photodetectors.
A BSI image sensor flips the metal layer and the substrate containing the array of photodetectors so that the array of photodetectors is positioned above the metal layer. Light is received by the photodetectors without having to pass through the metal layer. Image quality can be improved with a BSI image sensor because the photodetectors can detect more of the incident light.
Rolling shutter and global shutter are two different methods used by an image sensor to capture an image. With rolling shutter, all of the photodetectors in the image sensor do not capture the image simultaneously. Instead, different parts of the image sensor capture the image at different points in time. For example, all of the photodetectors in one row can accumulate charge during the same time period to capture an image, but the accumulation period for each row starts and ends at slightly different times. For example, the top row of photodetectors can be the first row to start accumulating charge and the first row to stop, with the start and stop times slightly delayed for each subsequent row of photodetectors. Images captured with rolling shutter can be subject to motion artifacts such as wobble, skew, and partial exposure because the rows of photodetectors capture the image at slightly different times.
With global shutter, all of the pixels accumulate charge at the same time. During a global shutter operation, the accumulated charge in the photodetectors is transferred simultaneously to storage regions located in the pixels before being read out of the image sensor. Typically, charge is read out of the pixels one row at a time. So storing the charge in storage regions allows the photodetectors to begin capturing the next image while the charge in the storage regions is read out of the image sensor.
FIG. 1 depicts a simplified pixel in a backside illuminated CMOS global shutter image sensor according to the prior art. The pixel 100 includes a sensing layer 102 that contains a photodetector 104 and a storage region 106. A metal layer 108 is located between a carrier wafer 110 and the sensing layer 102. The signal lines in the metal layer 108 are formed in a dielectric material so that the signal lines are electrically isolated from each other. A microlens 116 focuses light 118 onto the photodetector 104. The light shield 114 in layer 112 is positioned to shield the storage region 106 from the light 118 to prevent charge accumulation in the storage region 106. However, the distance between the storage region 106 and the light shield 114 can be two to three microns or more. This distance means light coming in a certain angles can still strike the storage region and cause unwanted charge to accumulate in the storage region 106. This unwanted charge is added to the photo-generated charge when the accumulated photo-generated charge in the photodetector 104 is transferred to the storage region 106 during a global shutter operation. The additional unwanted charge can cause image artifacts, and can result in an inaccurate image capture or representation of the imaged scene.